Is there any point to staging dual mod-cons?

Doug_7

The reason I would stage two ModCons is to get a more energy efficient overall operation and reduce maintenance costs.

You can broaden the modulating range of boilers when they are properly staged. If both boilers were 5:1 modulation, then when they are properly staged you will have a 10:1 overall modulation range.

At low total heat load only one staged boiler runs and the other boiler is shutdown, cold with no water flowing through it, zero pumping energy, zero flue losses and zero standby losses.

With smart-control two staged-boilers will pickup the heat load in a more efficient manner. They share the load when there is enough load to share. When there isn't enough load one boiler is shutdown and cold - which is very efficient.

I have seen too many un-staged boilers sitting on hot-standby for weeks and months, with hot water being pumped continuously through unfired boilers. This turns an atmospheric boiler heat exchanger into a radiator which sends heat up the flue and radiates heat through the jacket when it is not necessary.

The other argument with ModCons is that they are slightly more thermally efficient at low-load. Does this fuel saving offset the extra cost of pumping water and the extra heat losses ? I don't know.

Generally boilers are automatically staged to increase overall thermal efficiency and reduce operating hours and maintenance costs. With smart-staging controls, this occurs automatically 24/7/365 without human intervention. This saves money on fuel, pumping energy and maintenance. Well worth the price of a separate pump for each boiler.

What kind of ModCon's are you using ?

Doug

JohnNY

If I stage two mod-cons I have to pipe and pump them separately.

If they work concurrently at all times, I can use a single pump and simplify my piping.

Either way, my firing rate should be the same based on demand using the proper tN4 controls, no?

Alex "Stonehouse" Marx is wiring this for me, so if it's wrong I can definitely blame him.

Your thoughts?

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Brad White_203

We do it all the time

using even large commercial ModCons (2M and up per module). In this size plant (up to 10, 12, even 18 million BTUH input), we use isolation valves on unfired boilers, P/S piping in a variety of ways.

In your case, I see each piped in parallel to a common feeder and this goes into the main circuit. If each has it's own circulator with a flow-check, all the better.

The way we stage them (I am not intimately familiar with the tN4 details so this is general), this is what we do:

The first boiler fires to maintain radiation loop setpoint.

If and when the first boiler reaches the design limit (say 140 degrees, enough to ensure return water below dewpoint), the second boiler comes on-line and both fire together in parallel at a reduced rate.

The goal here is to keep the boiler temperatures as low as possible to maintain condensing and highest efficiency.

It is easy to "meet the load" by firing any one boiler to limit, so hence we fire in parallel.

I cannot say how to "make that happen" with the details that Alex brings to the table, but if Alex cannot make it happen, it cannot be done

JohnNY

Hmmmmmmmm.

So then why bother maintaining 140 when condensing is sort of the goal with mod/cons, Brad?
Is that just a design standard?

I'm having a hard time envisioning the benefit of staging two mod/cons simultaneously and equally EXCEPT if the demand exceeds the capacity of either single unit.

Would that be the only beneficial application?



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Brad White_203

The 140F

is just a coldest day design supply water setpoint, John.

My standard (adapted as necessary of course) is to use not higher than 140F supply water on the coldest day and return water 20 to 40 degrees colder than that. Most of my systems use a 140-100F supply-return temperature design basis. Anything under 120F will get me to condense.

As you correctly said, the reason we would twin them (or sextuplets, name your poison!), is for meeting capacity or for redundancy and standby capability should one fail. But within that, it is easy to over-fire and lose condensing, so you have to reign things in a bit.

For your case though, if any boiler has a maximum upset temperature each will not exceed, then I would stage them on knowing that neither will be higher than condensing temperature on the return side.

Many ways to do it- I think you will do well with this.